1. Field of the Invention
The present invention relates to a heating apparatus for heating a conveyed and introduced material to be heated at a heating portion and discharging the material.
More particularly, the present invention relates to a heating apparatus having mutually-pressure-welded heating member and pressure member to hold and convey a member to be heated by a nip portion formed between the heating member and the pressure member and heat the member to be heated.
Moreover, the present invention relates to an image forming apparatus such as a printer or copying machine having the heating apparatus as an image heating and fixing apparatus.
2. Description of Related Art
Hereafter, as an example, a fixing apparatus serving as a heating apparatus for heating and fixing an image in an image forming apparatus is described below.
A general fixing apparatus heats and pressure-fixes an unfixed image on a recording material (hereafter referred to as recording medium) surface by the heat of a heating member by pressure-welding the heating member and a pressure member, thereby forming a nip portion and paper-conveying the recording medium making the nip portion form and carry the unfixed image. As apparatus configurations, there are a film heating system and a roller heating system.
A fixing apparatus according to the film heating system uses an assembly having heating means (heating source) fixed and supported as a heating member and a flexible member sliding on the heating means. The heating member is constituted so as to form the heating means and fixing nip portion through the flexible member, hold and convey a recording medium for forming and carrying an unfixed image by the fixing nip portion and heat the recording medium by the heat of the heating means through the flexible member.
More specifically, the heating means generally uses the so-called ceramic heater which has a low thermal capacity and quick temperature rise. The flexible member uses a thin film (hereafter referred to as film) having a single layer configuration or compound layer configuration based on high-temperature resin or metal. Moreover, the fixing nip portion is formed by making the fixed and supported ceramic heater hold the film and pressure-welding the pressure roller to slide the film on the ceramic heater surface at the fixing nip portion. In the case of this configuration, the heat of the ceramic heater is supplied to a recording medium through the film by introducing the recording medium between the film of the fixing nip portion and the pressure roller and making the recording medium pass through the fixing nip portion together with the film.
In the case of the ceramic heater serving as heating means, an exothermic resistor which produces heat through electrification is set to one side of a thin-plate substrate mainly containing ceramic and a thermistor sensor (hereafter referred to as thermistor) serving as a temperature detector contacts with the other side of the substrate.
The fixing apparatus according to the roller heating system uses a cylindrical heat roller (hereafter referred to as fixing roller) having heating means such as a halogen heater at the inside or outside. The fixing nip portion is formed by pressure-welding a pressure roller serving as a heating member to the fixing roller and the recording medium is heated by the heat of the fixing roller by introducing a recording medium between the fixing roller and the pressure roller. A thermistor serving as a temperature detector contacts with the fixing roller.
In the case of the fixing apparatus, electrification from a commercial power supply to heating means is controlled so that the temperature detected by the thermistor reaches a temperature within a predetermined width from a target temperature and a heating temperature is adjusted.
Then, temperature control by the heating member in the continuous fixing step of a plurality of recording media in the fixing apparatus is described below by referring to FIG. 16. FIG. 16 is a graph showing transition of temperatures detected by a thermistor in the continuous fixing step of a plurality of recording media.
When an image forming apparatus having the above fixing apparatus starts an image forming process to a series of recording media constituted of N recording media, electrification from a commercial power supply to heating means is started and the temperature detected by the thermistor rises from a temperature a at the time of stop to temperature b and temperature c.
The electrification from the commercial power supply to the heating means is controlled so that the temperature detected by the thermistor is raised to a fixing temperature (heating temperature of recording medium) d immediately before the time T1 at which first recording medium enters the fixing nip portion. Then, the detected temperature is controlled so that it is maintained at a value within a predetermined width from the fixing temperature d from the period T1 to period T2 in which the recording medium is fixed.
Then, in the period before second recording medium is fixed after the first recording medium is fixed, the electrification from the commercial power supply to the heating means is controlled so that a temperature detected by the thermistor is maintained at a value within a predetermined width of the fixing temperature d or paper conveyance interval temperature e lower than the temperature d. By maintaining the temperature at the value within the predetermined width of the fixing temperature d or paper conveyance interval temperature e, it is possible to economize power consumption and prevent excessive temperature rise in a non-paper conveyance area at the outer periphery of a pressure roller.
The heating member is temperature-controlled as described above and heat is supplied from a heating member contacting with one side of a recording medium to heat the recording medium. Moreover, heat is also supplied from a pressure member contacting with the other side of the recording medium. Because the pressure member does not have heating means, the pressure member supplies the heat produced on the heating-member surface at a paper conveyance interval and accumulated to the recording medium under paper conveyance.
This control is disclosed in Japanese Patent Application Laid-Open Nos. S63-313182 and H06-149103.
However, the fixing apparatus controls the temperature of the heating member to a temperature equal to the fixing temperature d or the paper conveyance interval temperature e lower than the fixing temperature d.
The surface temperature of the pressure member is necessarily lowered compared to the temperature of the heating member surface heated at the fixing temperature d. Moreover, the temperature of the pressure member surface lowers by conveying heat quantity to the recording medium under paper conveyance.
As a result, the surface temperature of the pressure member considerably lowers compared to the temperature of the heating member surface and a large difference is produced in the heat quantity to be supplied to the surface of the recording medium when performing the heating and fixing operation at the fixing nip portion of the fixing apparatus.
When a recording medium is made of a material containing moisture such as paper, a difference is produced between moisture quantities evaporated from the front and back surfaces of the recording medium. Therefore, contraction and expansion values are changed at the front surface and back surface of the recording medium and there is a problem that warp and curl occur in the recording medium after fixed.
Particularly, when the environment in which an image forming apparatus or recording medium is put is a high-humidity environment and the recording medium contains much moisture and lacks in thickness and rigidity, curl becomes extreme.
When curl becomes extreme, it is impossible to systematically overlap or bind output recording media and the convenience of the output recording media is lost.
Moreover, when continuously outputting recording media, it is impossible to systematically mount the recording media on the discharge loading portion of an image forming apparatus and a problem occurs because output recording media are dropped and scattered or the discharge sequences are counterchanged. Furthermore, curled output recording media close the recording-medium discharge portion of the image forming apparatus, cause a bend or crease of a recording medium and stay to make an output of the image forming apparatus impossible.